A variety of materials have been used to absorb ultraviolet (UV) light and thus help to protect against UV radiation. For example, titanium dioxide (TiO2) is the most commonly used material for absorbing UV light. However, titanium dioxide is only effective in absorbing UVB, but not UVA, which is more difficult to be protected among all UV wavelengths and is known as a leading cause of human skin cancer.
In order to solve the aforementioned problem, materials that are effective in shielding UV light (i.e. reflecting UV light), such as zinc oxide that is capable of shielding both UVA and UVB, has been utilized to protect against UV radiation. However, due to the high polarity of zinc oxide, its powdered form is often agglomerated and hardly dispersed. One way to solve such problem is to modify the physicochemical properties of zinc oxide powder.
For example, Taiwanese Invention patent No. 1546260 discloses core-shell zinc oxide particles having improved dispersibility, which includes a core of calcium carbonate material and a shell layer of zinc oxide flakes. US patent publication No. 2007/0072959 A1 discloses composite particles in which zinc oxide is formed on a particle core that includes silica, clay, talc, mica, activated carbon and carbon black, and/or in which the zinc oxide is co-precipitated as a coating or intermixed with other particles.
Although the conventional core-shell zinc oxide particles as mentioned above have improved dispersibility, none of the them has been used as a ultraviolet light protection material. Therefore, the applicants endeavor to prepare core-shell zinc oxide particles having excellent UV light-shielding effect.